Question

Meroblastic cleavage is ________division
A) Horizontal
B) Partial/parietal
C) Total
D) Spiral

Answer 1

Hint: The division of blastomeres inside the egg depends on the amount of yolk present inside the egg cells. The organisms have larger eggs that show partial division while animals with a small size of egg show total division.

Complete answer:
To answer this question, we have to know about the meroblastic cleavage. In animals like birds and reptiles, the incomplete cleavage in mesolecithal and telolecithal egg takes place. The cleavage is depending on the amount of yolk found inside the egg, either it is holoblastic that means total cleavage or meroblastic that means partial cleavage. In meroblastic cleavage only one portion of the egg get divided as the large amount of yolk in the cell that cleave do not penetrate the yolk. The meroblastic cleavage is further divided into discoidal cleavage and superficial cleavage. The discoidal cleavage is a type of meroblastic cleavage where the cytoplasmic cell divisions are not allowed to the animal pole, the outcome of which a disk of cells is formed that confined in the top of the yolk. This type of cleavage is observed in reptiles, fishes, and birds egg, as they consists large amount of yolk that do not undergo cleavage. The superficial cleavage is another type of meroblastic cleavage where the mitosis takes place without cytokinesis the outcome of which any nuclei migrated towards the periphery of the egg. This type of cleavage is observed in insects as they hav centrolecithal eggs that mean the egg yolk is found in the center of the egg cell.
Now, from the above discussion we can conclude that the meroblastic cleavage shows partial or parietal division because of the presence of a large amount of yolk. But the complete division is observed in isolecithal or microlecithal eggs. This type of cleavage is found in amphibians, echinoderms, flatworms, annelids, mammals, nematodes etc as the egg cell has a moderate amount of yolk.

Thus, the correct answer is ‘B’. partial/parietal.

Note:Equal cleavages of the blastomeres are observed in the holoblastic cleavage that means the division takes place equally. The holoblastic cleavage also shows unequal cleavage, it produces blastomeres of unequal size.